Coronavirus causes disease in man and animals. This group of single-stranded, positive-sense, enveloped RNA viruses causes a range of serious gastrointestinal and upper respiratory tract infections. The overall objective of this grant application is to learn how this viral pathogen assembles and interacts with its host in vitro, by using mouse hepatitis virus (MHV) as a coronavirus model system. Coexpression of MHV M and E proteins results in the production of virus-like particles (VLP). The significance of putative M protein-E protein binding and putative M protein oligomerization at MHV budding sites for VLP production will be examined in cells coexpressing M and E proteins. Subsequently the significance of these protein interactions in MHV envelope formation will be evaluated in MHV-infected cells. The 69 nt-long packaging signal (PS) is necessary and sufficient for MHV RNA packaging. MHV nucleocapsid, which is made up of MHV RNAs containing the PS and N protein, binds MHV M protein; this specific interaction most probably determines specific packaging of MHV RNA into MHV particles. We will study the status of N protein binding to MHV RNAs containing the PS and those lacking the PS to know whether or not the PS is the nucleation site for nucleocapsid formation. Expression of non-MHV transcripts containing the PS in MHV-infected cells will be used to learn how important nucleocapsid structure is for RNA packaging. Also how various RNA transcripts containing the PS affect N protein-M protein binding will be examined. Finally, the M protein region that interacts with nucleocapsid will be identified. Mature cytoplasmic 28S ribosomal RNA (rRNA) undergoes a novel specific cleavage in MHV-infected cells. W will examine the translational function of ribosomal subunits containing the cleaved 28S rRNA to test whether the MHV induced 28S rRNA cleavage alters the state of the ribosomes and affects host protein synthesis in infected cells. In a separate study on translational control in MHV-infected cells, we will identify a cis-acting MHV RNA element(s) that suppresses translation of MHV gene I proteins late in infection. MHV infection in the 17C1-1 cell line induces apoptosis, which appears to involve two major caspase activation pathways. We will clarify the mechanisms of the two caspase activation pathways, and will further explore the possibility that an MHV protein(s) is involved in a caspase activation pathway in a way that is equivalent to that of a host protein.